Hitherto, inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide have been prevailingly applied to the photosensitive layer of photoreceptor for electrophotography. However, selenium and cadmium sulfide have to be recovered as poisonous substance, and further selenium is poor in heat resistance as it is crystallized when heated. Cadmium sulfide and zinc oxide, are poor in moisture resisitance, and zinc oxide is also deficient in printing endurance. Thus, efforts have been made for researching and developing a novel photosenstive material and novel photoreceptor.
A remarkable progress has been made recently in the studies on the use of organophotoconductive materials for the photosensitive layer of electrophotographic photoreceptor, and some of such organophotoconductive materials have been put to practical use.
The organophotoconductive materials have many advantages over the inorganic. For example, they are light in weight, it is by far easier to form a film and to produce a photosensitive layer by making use of them, and among them some are capable of producing a transparent photoreceptor.
Recently, the so-called function divided type of photoreceptor--in which the generation and transport of charge carriers are separately effected by different compounds--has become the main object of development since this type of photoreceptor is most effective in attaining high sensitivity, and some orgnophotoreceptors of such type have already been put to practical use.
As the transport material for the charge carrier, there is known either a high molecular photoconductive compound such as polyvinyl carbazole or a low molecular photoconductive compound dispersed or dissolved in a binder polymer.
The organic low molecular photoconductive compound can produce the photoreceptor excellent in mechanical properties since it is possible to select a polymer having excellent film-forming properties, flexibility, adhesivness, etc., as a binder for the compound. However, it is difficult to find out such a low molecular compound suited for making a highly sensitive photoreceptor.